1. Field of the Invention
The present invention relates to a process for recovering unreacted ammonia and carbon dioxide from a urea synthesis effluent obtained by reacting carbon dioxide and ammonia and, more particularly, to a process for effectively recovering heat and for recovering ammonia, by use of the recovered heat, from condensed water which is obtained by condensing water vapor evaporated upon concentration of an aqueous urea solution.
2. Description of the Prior Art
In the production of urea by the so-called total solution recycle process, starting ammonia and carbon dioxide are reacted with each other under high temperature and high pressure conditions conventionally known and recognized by those skilled in the art as urea synthesis conditions to obtain a urea synthesis effluent. The urea synthesis effluent containing unreacted ammonia and ammonium carbamate is generally subjected to a plurality of decomposition stages by stripping or distillation using stepwise reduced pressure to decompose the unreacted materials and to separate them from the urea synthesis effluent at each stage as an off-gas consisting of a mixed gas of ammonia, carbon dioxide and water vapor. The off-gases separated in the respective stages are each absorbed in an absorbent, which is fed from the absorption zone corresponding with the stripping or distillation zone of the next lower decomposition stage, under substantially the same pressure as the pressure in each of the stages where the off-gas has been separated. The resulting absorbates are used as absorbents in the respective higher pressure stages. The off-gases separated in the respective stages are thus absorbed by turn and the final absorbate discharged from the absorption zone of the highest pressure stage is increased in pressure up to the urea synthesis pressure and recycled to the urea synthesis zone. In the separation zones of the respective decomposition stages, the unreacted ammonium carbamate is thermally decomposed and separated into ammonia and carbon dioxide by indirect heating with steam, etc. The major part of the amount of heat required for the urea production plant is consumed for this heating. There is accordingly a serious problem of how to effectively reduce the amount of heat required for recovering the unreacted materials.
The decomposition reaction of the unreacted ammonium carbamate is highly endothermic. On the other hand, when the mixed gas containing ammonia and carbon dioxide which is obtained as a result of the decomposition of the ammonium carbamate is absorbed in an absorbent, a great amount of heat of absorption is liberated. In order to recover this heat of absorption, it is advantageous to effect the absorptions in the respective stages under conditions of temperature and pressure as high as possible. Where the heat of absorption is recovered in the form of steam, the steam is preferably high in temperature and pressure so that it can be effectively employed for various uses.
Attempts toward recovering the excess of heat in the form of steam of good quality having a urea synthesis temperature and a pressure of 5 kg/cm.sup.2 (gauge) or more have heretofore been made and a number of methods have been proposed. In one such method, a recovered ammonium carbamate solution, holding, as sensible heat, the heat of absorption liberated in a high pressure absorption stage, is fed to a urea synthesis zone as it is without recovering the heat in each absorption stage and then the heat of absorption is recovered in a heat exchanger disposed in the urea synthesis zone. In the method described in U.S. Pat. No. 3,944,605, the heat energy liberated in a high pressure absorption zone is transferred to liquid ammonia, the pressure of which is increased to urea pressure, by means of heating the liquid ammonia in a heat exchanger provided in the high pressure absorption zone, and then the excess amount of heat for the urea synthesis is recovered in a heat exchanger disposed in the urea synthesis zone or in a heat recovery zone ahead of the urea synthesis zone, etc. Although these methods are advantageous in obtaining high temperature and high pressure steam, they have the common disadvantage in that the heat exchange is conducted in the urea synthesis zone under high temperature and high pressure conditions and in a highly corrosive fluid, so that the heat exchanger is essentially required to have high resistances to pressure, heat and corrosion, increasing the cost of equipment to a considerable extent.
As described hereinbefore, in the urea concentration step, a small amount of ammonia is discharged together with the condensed water derived from water vapor generated upon the concentration. This previously has not been a serious problem due to its small amount. In recent years, however, intense interest has been shown towards the discharged ammonia due to scaling up of production of the urea production plants. In conventional mediumsized apparatus, it has been almost impossible to economically recover small amounts of ammonia and carbon dioxide from an aqueous urea solution in a finishing step, so that the ammonia and carbon dioxide are discarded together with the waste water. With a large-scale urea production plant, however, the discarded ammonia and carbon dioxide are not negligible in amount. That is, with a large-scale plant, even an extremely small content of ammonia in the waste water reaches a substantial amount since the total amount of waste water becomes very great, causing environmental pollution due to contamination of rivers or seas therewith. There is accordingly a realistic need for the development of an improved process for recovering ammonia by which the loss of valuable ammonia is prevented and the pollution problem is solved. In order to meet this requirement, there has been proposed a process for separating and recovering ammonia and carbon dioxide from an aqueous solution containing traces of ammonia and carbon dioxide by subjecting the aqueous solution to rectification or a stripping treatment using water vapor to separate therefrom a mixed gas composed of ammonia, carbon dioxide and water vapor, and recovering by absorption the mixed gas along with an off-gas obtained by low pressure decomposition of unreacted ammonium carbamate. However, this process has disadvantages in that the heat energy of the steam consumed in the rectification or the stripping treatment is lost during cooling and absorption operations for the mixed gas and that additional cooling water is necessary to remove the heat of condensation of the water vapor. In addition, the steam content in the mixed gas is relatively great, so that the absorbate obtained by absorbing the mixed gas is diluted to a considerable extent. Introduction of such diluted solution into the urea synthesis zone results in reduction in conversion in the urea synthesis reaction. In order to overcome these disadvantages, there has been provided in U.S. patent application Ser. No. 651,569 filed Jan. 22, 1976, a process which comprises subjecting an aqueous solution containing traces of ammonia and carbon dioxide to rectification, and feeding the resulting mixed gas composed of ammonia and carbon dioxide and water vapor to a low pressure decomposition zone for unreacted ammonia carbamate to recover ammonia and carbon dioxide together with the heat required for the rectification. The present invention contemplates providing a process for recovering these unreacted materials and heat in a more efficient manner.
In urea synthesis, it is a common practice to indirectly heat the low pressure decomposition zone for separating unreacted ammonium carbamate by means of a steam heater provided in said decomposition zone. For said heating by indirect heat exchange, the steam must necessarily have a pressure above 5 kg/cm.sup.2, it being almost impossible to use steam of a lower pressure. Japanese Patent Publication No. 15015/1970 describes a process for thermally decomposing and stripping off unreacted ammonium carbamate by directly blowing low pressure steam into the unreacted ammonium carbamate containing urea synthesis effluent in a low pressure decomposition zone. However, if the aqueous dilute solution containing ammonia and carbon dioxide which has been produced upon concentration of an aqueous urea solution is discarded as it is similar to conventional practice, the unreacted ammonium carbamate which has not been separated and recovered in the low pressure decomposition zone is lost. On the other hand, in order to separate substantially all of the ammonia and carbon dioxide by steam stripping in the low pressure decomposition zone, the resulting ammonia- and carbon dioxide-free urea synthesis effluent is inescapably brought into contact with hot water vapor in the vacinity of the bottom of the decomposition zone or tower, whereupon hydrolysis of urea undesirably takes place. This is one of the reasons why the process has not been put into commercial practice even though it has the prominent advantage that low pressure steam can be used.